feat: support backfill_rate_limit for source backfill

[xxchan]

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What's changed and what's your intention?

fix #19297

This is a missing feature. We support source_rate_limit for shared source executor, but cannot rate limit source backfill before this PR.

Checklist

• [ ] I have written necessary rustdoc comments
• [ ] I have added necessary unit tests and integration tests
• [ ] I have added test labels as necessary. See details.
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Release note

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[xxchan]

• #19466
• #19445 👈 (View in Graphite)
• #19443
• main

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[tabVersion]

Please also leave some doc on how to use the source_rate_limit and backfill_rate_limit from the user's perspective. It is really confusing.

[xxchan]

https://github.com/risingwavelabs/risingwave-docs-new/pull/62

[xxchan]

Merge activity

• Nov 20, 3:57 PM GMT+8: A user started a stack merge that includes this pull request via Graphite.
• Nov 20, 3:58 PM GMT+8: Graphite couldn't merge this pull request because a downstack PR #19443 failed to merge.
• Nov 20, 6:53 PM GMT+8: Graphite rebased this pull request after merging its parent, because this pull request is set to merge when ready.
• Nov 20, 7:34 PM GMT+8: Graphite couldn't merge this PR because it failed for an unknown reason (This repository has GitHub's merge queue enabled, which is currently incompatible with Graphite).
• Nov 20, 7:35 PM GMT+8: Graphite couldn't merge this PR because it failed for an unknown reason (This repository has GitHub's merge queue enabled, which is currently incompatible with Graphite).